A drift detector is a semiconductor radiation detector in which a transversal electric field created inside a block of semiconductor material drives radiation-induced signal charges to a collection electrode. Widely known is the SDD or silicon drift detector, in which the semiconductor material is high-purity silicon with a very low leakage current, and the transversal electric field is created with concentric ring- or arc-like field electrodes on its surface. The collection electrode is an anode located close to the common center point of the electrode rings.
Two basic electrode layouts are known for SODS. The more traditional SDDs are ring-shaped, with the collection anode located at the center. More recently there have been introduced the so-called droplet-shaped SDDs that have a wide and a pointed end. The electrode rings spread out across the large end, while the collection anode is located relatively close to the narrow end.
An amplifier, which typically comprises a field-effect transistor (FET), is frequently combined with an SDD either by building it directly into the same piece of semiconductor material or by combining it with the SDD chip, for example by bump bonding to contact pads on its surface. The electrode rings in the SDD are of the p+ semiconductivity type and they are made by ion implantation. An electric insulator layer, typically made of silicon dioxide, covers that side of the SDD chip that carries the electrode rings, the collection anode, and the amplifier (if any).
Although the task of a radiation detector is to absorb radiation, it suffers from adverse effects of absorbed ionizing radiation over time like all semiconductor devices. As an example of radiation-induced ageing, the surface-generated current at the SiO2—Si interface in between the ion-implanted p+ rings is increased by radiation. Also other kind of radiation-induced ageing, which does not appear to be related to said surface current, develops over time, weakening the measurement accuracy and eventually leading to the need to replace the whole detector.